PALEONTOLOGY, A TOOL TO RESOLVE LATE PALEOZOICSTRUCTURAL AND DEPOSITIONAL HISTORIES

摘要

Abundant and accurate paleontologic ages and dates help to refine the magnitude and scale of structural and sedimentary events. They are critical in understanding the geologic history of Pennsylvanian and Permian strata of southeastern Arizona, southern New Mexico, and west Texas, U.S.A. A well-established fusulinacean zonation in these intervals is one of several well-studied fossil zonations used for successful petroleum exploration in this area. The results have led to the timing of movements of large structural blocks and show the progressive, but not uniform, movements on different blocks at different times caused by reactions to stages in the Marathon–Ouachita orogeny to the south and east. Superimposed on this structural history are the patterns of Pennsylvanian and Permian cyclic deposits resulting from repeated sea-level fluctuations thought to be mainly of glacioeustatic origin. The magnitude of these sea-level fluctuations, and the lateral extent of each transgression onto various structures, can be used as a gauge in determining the height of various structures at different times. Inasmuch as the Pennsylvanian and Permian sea levels are well dated by fossils, it is possible to identify each sea-level fluctuation and to compare it to a "standard" cratonic sea-level curve on the more stable parts of the craton. Paleontologically well-dated structural movements and the timing and magnitude of sea-level fluctuations help identify various types of petroleum reservoirs. They include lowstand-wedge carbonate reefal buildups in lower slope to basinal settings, multiple thin sheet-like porous deposits on platform crests, karsted platform-edge facies, and coarse shelf carbonate debris reworked into porous platform slope and basin deposits. In addition a variety of other shelf-margin features, including variously large-sized slide blocks (many more than 5 km on a side) of shallow shelf carbonates were displaced by faulting or gravity sliding during the Pennsylvanian and/or Permian into basinal settings.